Ok...
I did a talk a while back on exotic 'atoms' - positronium was one of them....

If you put positronium in an excited state or somehow manipulate it so that it is bound but the wavefunctions of the electron and positron never overlap, then yes- I guess that could work. The trick would be keeping them all in that state all the time. (I still don't believe the cost estimate to make that many positrons)...

It does have the advantage that you have the two particles you need to annihilate right there together.... you could somehow transport the neutral positronium to the reaction chamber and then stimulate it into a state where the wavefunctions of the two particled overlap, they could then annihilate and release their energy...

Interesting, I am am glad you followed that up...